1. Field of the Invention
The present invention provides a polarization converting element group, and more particularly, to a polarization converting element group for a projection apparatus.
2. Descriptions of the Related Art
Over recent years, digital light processing (DLP) projection systems have become mainstream in the projection apparatus market because of the features, such as high brightness, vivid colors, rapid response and a light weight. In addition to conventional DLP projection systems for displaying flat images, there are also DLP projection systems for use as stereoscopic displaying projection systems.
Stereoscopic displaying projection systems may generally be categorized into eyeglass stereoscopic displaying projection systems and auto-stereoscopic displaying projection systems. Because of low manufacturing costs thereof, eyeglass stereoscopic displaying projection systems, which can be subdivided into active and passive eyeglasses, have been widely used.
Active eyeglasses, which are also known as shutter eyeglasses, operate projection apparatuses at different time sequences. Specifically, a grayscale picture is inserted between two frames by the projection system, and while receiving the grayscale picture, the active eyeglasses switch between the left-eye shutter and the right-eye shutter. For example, when a grayscale picture is received, the active eyeglasses turns on the left-eye shutter and then, a left-eye view-angle image is provided by the projection apparatus so that the user's left eye will receive the left-eye view-angle image. In this way, the left-eye and right-eye view-angle images are provided in alternating time sequences by the projection apparatus. However, because of the switching action between the two shutters, there is always one eye that is covered, and this shortens the time duration in which each eye can view the screen. Consequently, the brightness perceived by each eye of the viewer is reduced by at least 50%. Furthermore, the active eyeglasses adjust the light transmittance of the individual eyeglass lenses by use of liquid crystals, which makes them more expensive than passive ones. The switching structure and the associated control circuit also add to the weight of the eyeglasses, which makes the user uncomfortable. In other words, DLP projection systems using active eyeglasses are inevitably costly and are inconvenient to use.
Accordingly, it is important to provide a stereoscopic projection system that can work with passive eyeglasses and that has a low cost, a compact optical arrangement, a miniaturized volume and increased brightness.